Carbon is a preferred product of biomass pyrolysis at a moderate temperature. Thermodynamic equilibrium calculation shows that the char yield of most biomass may not exceed 35%. Table 3.6 gives the theoretical equilibrium yield of biomass at different temperatures. Assuming that cellulose represents bio­mass, the stoichiometric equation for production of charcoal (Antal, 2003) may be written as

C6Hi<A ^ 3.74C + 2.65H2O +1.17CO2 +1.08CH4 (3.11)

Charcoal production from biomass requires slow heating for a long duration but at a relatively low temperature of around 400 °C. An extreme example of a pyrolysis or carbonization is in the coke oven in an iron and steel plant, which pyrolyzes (carbonizes) coking coal to produce hard coke used for iron extrac­tion. This is an indirectly heated fixed-bed pyrolyzer that operates at a tempera­ture exceeding 1000 °C and for a long period of time to maximize gas and solid coke production.

With growing evidence of global warming, the need to reduce human-made greenhouse gas emissions is being recognized. Emission of other air pollutants, such as NO2, SO2, and Hg, is no longer acceptable, as it was in the past. In

elementary schools and in corporate boardrooms, the environment is a major issue, and it has been a major driver for gasification for energy production. Biomass has a special appeal in this regard, as it makes no net contribution to carbon dioxide emission to the atmosphere. Regulations for making biomass economically viable are in place in many countries. For example, if biomass replaces fossil fuel in a plant, that plant earns credits for CO2 reduction equiva­lent to what the fossil fuel was emitting. These credits can be sold on the market for additional revenue in countries where such trades are in practice.

Carbon Dioxide Emission

When burned, biomass releases the CO2 it absorbed from the atmosphere in the recent past, not millions of years ago, as with fossil fuel. The net addition of CO2 to the atmosphere through biomass combustion is thus considered to be zero.

Even if the fuel is not carbon-neutral biomass, CO2 emissions from the gasification of the fuel are slightly less than those from its combustion on a unit heat release basis. For example, emission from an IGCC plant is 745 g/ kWh compared to 770 g/kWh from a combustion-based subcritical pulverized — coal (PC) plant (Termuehlen and Emsperger, 2003, p. 23). Sequestration of CO2 is becoming an important requirement for new power plants. On that note, a gasification-based power plant has an advantage over a conventional combustion-based PC power plant. In an IGCC plant, CO2 is more concentrated in the flue gas, making it easier to sequestrate than it is in a conventional PC plant. Table 1.5 compares the emissions from different electricity-generation technologies.

Sulfur Removal

Most virgin or fresh biomass contains little to no sulfur. Biomass-derived feedstock such as municipal solid waste (MSW) or sewage sludge does contain sulfur, which requires limestone for the capture of it. Interestingly, such derived feedstock also contains small amounts of calcium, which intrinsically aids sulfur capture.

Gasification from coal or oil has an edge over combustion in certain situa­tions. In combustion systems, sulfur in the fuel appears as SO2, which is rela­tively difficult to remove from the flue gas without adding an external sorbent. In a typical gasification process 93 to 96% of the sulfur appears as H2S with the remaining as COS (Higman and van der Burgt, 2008, p. 351). We can easily extract sulfur from H2S by absorption. Furthermore, in a gasification plant we can extract it as elemental sulfur, thus adding a valuable by-product for the plant.

Nitrogen Removal

A combustion system firing fossil fuels can oxidize the nitrogen in fuel and in air into NO, the acid rain precursor, or into N2O, a greenhouse gas. Both are difficult to remove. In a gasification system, nitrogen appears as either N2 or NH3, which is removed relatively easily in the syngas-cleaning stage.

Nitrous oxide emission results from the oxidation of fuel nitrogen alone. Measurement in a biomass combustion system showed a very low level of N2O emission (Van Loo and Koppejan, 2008, p. 295).

Dust and Other Hazardous Gases

Highly toxic pollutants like dioxin and furan, which can be released in a com­bustion system, are not likely to form in an oxygen-starved gasifier. (This observation is disputed by some.) Particulate in the syngas is also reduced significantly by multiple gas cleanup systems, including a primary cyclone, scrubbers, gas cooling, and acid gas-removal units. These reduce the particulate emissions by one to two orders of magnitude (Rezaiyan and Cheremisinoff, 2005, p. 15).

As mentioned earlier, pyrolysis involves a breakdown of large complex mole­cules into several smaller molecules. Its product is classified into three principal types:

• Solid (mostly char or carbon)

• Liquid (tars, heavier hydrocarbons, and water)

• Gas (CO2, H2O, CO, C2H2, C2H4, C2H6, C6H6, etc.)

The relative amounts of these products depend on several factors including the heating rate and the final temperature reached by the biomass.

The pyrolysis product should not be confused with the "volatile matter” of a fuel as determined by its proximate analysis. In proximate analysis, the liquid and gas yields are often lumped together as "volatile matter,” and the char yield as "fixed carbon.” Since the relative fraction of the pyrolysis yields depends on many operating factors, determination of the volatile matter of a fuel requires the use of standard conditions as specified in test codes such as ASTM D-3172 and D-3175. The procedure laid out in D-3175, for example, involves heating a specified sample of the fuel in a furnace at 950 °C for seven minutes to measure its volatile matter.

Solid

Char is the solid yield of pyrolysis. It is primarily carbon (~85%), but it can also contain some oxygen and hydrogen. Unlike fossil fuels, biomass contains very little inorganic ash. The lower heating value (LHV) of biomass char is about 32 MJ/kg (Diebold and Bridgwater, 1997), which is substantially higher than that of the parent biomass or its liquid product.

Liquid

The liquid yield, known as tar, bio-oil, or biocrude, is a black tarry fluid con­taining up to 20% water. It consists mainly of homologous phenolic com­pounds. Bio-oil is a mixture of complex hydrocarbons with large amounts of oxygen and water. While the parent biomass has an LHV in the range of 19.5 to 21 MJ/kg dry basis, its liquid yield has a lower LHV, in the range of 13 to 18 MJ/kg wet basis (Diebold et al., 1997).

Bio-oil is produced by rapidly and simultaneously depolymerizing and fragmenting the cellulose, hemicellulose, and lignin components of biomass. In a typical operation, the biomass is subjected to a rapid increase in tempera­ture followed by an immediate quenching to “freeze” the intermediate pyrolysis products. Rapid quenching is important, as it prevents further degradation, cleavage, or reaction with other molecules (see Section 3.4.2 for more details).

Bio-oil is a microemulsion, in which the continuous phase is an aqueous solution of the products of cellulose and hemicellose decomposition, and small molecules from lignin decomposition. The discontinuous phase is largely com­posed of pyrolytic lignin macromolecules (Piskorz et al., 1988). Bio-oil typi­cally contains molecular fragments of cellulose, hemicellulose, and lignin polymers that escaped the pyrolysis environment (Diebold and Bridgwater, 1997). The molecular weight of the condensed bio-oil may exceed 500 Daltons (Diebold and Bridgwater, 1997). Compounds found in bio-oil fall into the fol­lowing five broad categories (Piskorz et al., 1988):

• Hydroxyaldehydes

• Hydroxyketones

• Sugars and dehydrosugars

• Carboxylic acids

• Phenolic compounds

Gas

Primary decomposition of biomass produces both condensable gases (vapor) and noncondensable gases (primary gas). The vapors, which are made of heavier molecules, condense upon cooling, adding to the liquid yield of pyro­lysis. The noncondensable gas mixture contains lower-molecular-weight gases like carbon dioxide, carbon monoxide, methane, ethane, and ethylene. These do not condense on cooling. Additional noncondensable gases produced through secondary cracking of the vapor (see Section 3.4.2) are called secondary gases. The final noncondensable gas product is thus a mixture of both primary and

secondary gases. The LHV of primary gases is typically 11 MJ/Nm3, but that of pyrolysis gases formed after severe secondary cracking of the vapor is much higher: 20 MJ/Nm3 (Diebold and Bridgwater, 1997). Table 3.1 compares the heating values of pyrolysis gas with those of bio-oil, raw biomass, and two fossil fuels.

Botanical biomass is formed through conversion of carbon dioxide (CO2) in the atmosphere into carbohydrate by the sun’s energy in the presence of chlo­rophyll and water. Biological species grow by consuming botanical or other biological species.

Plants absorb solar energy in a process called photosynthesis (Figure 2.1). In the presence of sunlight of particular wavelengths, green plants break down water to obtain electrons and protons and use them to turn CO2 into glucose

(represented by CHmO„), releasing O2 as a waste product. The process may be represented by this equation (Hodge, 2010, p. 297):

Living plant + CO2 + H2O + Sunlight Chlorophyl1 > (CHmO„) (21)

+ O2 — 480 kJ/mol ( . )

For every mole of CO2 absorbed into carbohydrate or glucose in biomass, 1 mole of oxygen is released. This oxygen comes from water the plant takes from the ground or the atmosphere (Klass, 1998, p. 30). The chlorophyll pro­motes the absorption of carbon dioxide from the atmosphere, adding to the growth of the plant. Important ingredients for the growth of biomass are:

• Living plant

• Visible spectrum of solar radiation

• Carbon dioxide

• Chlorophyll (serving as catalyst)

• Water

The chemical energy stored in plants is then passed on to the animals and to the human that take the plants as food. Animal and human waste also contribute to biomass.

An important question for designers is whether a pyrolyzer can meet its own energy needs or is dependent on external energy. The short and tentative answer is that a pyrolyzer as a whole is not energy self-sufficient. The reaction heat is inadequate to meet all energy demands, which include heat required to raise the feed and any inert heat-transfer media to the reaction temperature, heat consumed by endothermic reactions, and heat losses from the reactor. In most cases it is necessary to burn the noncondensable gases and the char produced to provide the heat required. If that is not adequate, other heat sources are necessary to supply the energy required for pyrolysis. The following section discusses the heat requirement of reactions taking place in a pyrolyzer.

The dehydration (reaction II) process is exothermic, while depolymerization (reaction III) and secondary cracking (reaction IV) are endothermic (Bridgwa­ter et al., 2001). Among reactions between intermediate products of pyrolysis, some are exothermic and some are endothermic. In general, pyrolysis of hemi — cellulose and lignin is exothermic. Cellulose pyrolysis is endothermic at lower temperatures (<400-450 °C), and it becomes exothermic at higher temperatures owing to the following exothermic reactions (Klass, 1998).

CO + 3H2 ^ CH4 + H2O — 226 kJ/gmol
CO + 2H2 ^ CH3OH -105 kJ/gmol
0.17 C6H10O5 ^ C + 0.85 H2O — 80 kJ/gmol
CO + H2O ^ CO2 + H2 — 42 kJ/gmol

(All equations refer to a temperature of 1000 K, and C6Hi0O5 represents the cellulose monomer.)

For this reason a properly designed system initially requires external heat only until the required temperature is reached.

Char production from cellulose (Eq. 3.8) is slightly exothermic. However, at a higher temperature, when sufficient hydrogen is produced by reaction (Eq. 3.9), other exothermic reactions (Eqs. 3.6 and 3.7) can proceed. At low temperatures and short residence times of volatiles, only endothermic primary reactions are active (heat of reaction -225 kJ/kg), while at high temperatures exothermic secondary reactions (heat of reaction 20 kJ/kg) are active (Blasi, 1993).

In conclusion, for design purposes one may neglect the heat of reaction for the pyrolysis process, but it is necessary to calculate the energy required for vaporization of products and for heating feedstock gases to the pyrolysis tem­perature (Boukis et al., 2007).

The ternary diagram (Figure 2.12) is not a tool for biomass classification, but it is useful for representing biomass conversion processes. The three corners of the triangle represent pure carbon, oxygen, and hydrogen—that is, 100% con­centration. Points within the triangle represent ternary mixtures of these three substances. The side opposite to a corner with a pure component (C, O, or H) represents zero concentration of that component. For example, the horizontal

base in Figure 2.12 opposite to the hydrogen corner represents zero hydrogen— that is, binary mixtures of C and O.

A biomass fuel is closer to the hydrogen and oxygen corners compared to coal. This means that biomass contains more hydrogen and more oxygen than coal contains. Lignin would generally have lower oxygen and higher carbon compared to cellulose or hemicellulose. Peat is in the biomass region but toward the carbon corner, implying that it is like a high-carbon biomass. Peat, inciden­tally, is the youngest fossil fuel formed from biomass.

Coal resides further toward the carbon corner and lies close to the oxygen base in the ternary diagram, suggesting that it is very low in oxygen and much richer in carbon. Anthracite lies furthest toward the carbon corner because it has the highest carbon content. The diagram can also show the geological evolution of fossil fuels. With age the fuel moves further away from the hydro­gen and oxygen corners and closer to the carbon corner.

As mentioned earlier, the ternary diagram can depict the conversion process. For example, carbonization or slow pyrolysis moves the product toward carbon through the formation of solid char; fast pyrolysis moves it toward hydrogen and away from oxygen, which implies higher liquid product. Oxygen gasifica­tion moves the gas product toward the oxygen corner, while steam gasification takes the process away from the carbon corner. The hydrogenation process increases the hydrogen and thus moves the product toward hydrogen.

Torrefaction, a process different from carbonization, is a mild pyrolysis process carried out in a temperature range of 230 to 300 °C in the absence of oxygen. This thermal pretreatment of biomass improves its energy density, reduces its oxygen-to-carbon (O/C) ratio, and reduces its hygroscopic nature. During this process the biomass dries and partially devolatilizes, decreasing its mass while largely preserving its energy content. The torrefaction process removes H2O and CO2 from the biomass. As a result, both the O/C and the H/C ratios of the biomass decrease. In raw biomass, high oxygen content prompts its over­oxidation during gasification, increasing the thermodynamic losses of the process. Torrefaction could reduce this loss by reducing the oxygen in the biomass. Torrefaction also increases the relative carbon content of the biomass. The properties of a torrefied wood depends on torrefaction temperature, time, and on the type of wood feed.

A popular example of torrefaction is the process of roasting coffee beans. As the green beans are heated to 200 to 300 °C, their surface darkens (www. coffeeresearch. org/coffee). Figure 3.10 contains photographs of rice husk, peanut husk, bagasse, and water hyacinth before and after torrefaction. The color change is present in all biomass but to different degrees.

Torrefaction also modifies the structure of the biomass, making it more friable or brittle. This is caused by the depolymerization of hemicellulose. As a result, the process of size reduction becomes easier, lowering its energy con­sumption and the cost of handling. This makes it easier to co-fire biomass in a pulverized-coal fired boiler or gasify it in an entrained-flow reactor.

Torrefaction causes some reduction in the energy content of the biomass because of partial devolatilization, but given the much higher reduction in mass, the energy density of the biomass increases. Table 3.7 shows an example of torrefaction. Here, we note that by losing only 11 to 17% energy, the biomass

(bagasse) lost 31 to 38% of its original mass. Thus, there is a 29 to 33% increase in energy density (energy per unit mass) of the biomass. This increases its higher heating value (HHV) to about 20 MJ/kg. Even if we take into account the energy used in the torrefaction process, we can see from Table 3.7 that there is a net rise in the energy density of the fuel.

Another special feature of torrefaction is that it reduces the hygroscopic property of biomass; therefore, when torrefied biomass is stored, it absorbs less moisture than that absorbed by fresh biomass. For example, while raw bagasse absorbed 186% moisture when immersed in water for two hours, it absorbed only 7.6% moisture under this condition after torrefying the bagasse for 60 minutes at 250 °C (Pimchua et al., 2009). The reduced hygroscopic (or enhanced hydrophobic) nature of torrefied biomass mitigates one of the major shortcom­ings for energy use of biomass.

The sociopolitical benefits of biomass are substantial. For one, biomass is a locally grown resource. For a biomass-based power plant to be economically viable, the biomass needs to come from within a certain distance from it. This means that every biomass plant can prompt the development of associated industries for biomass growing, collecting, and transporting. Some believe that a biomass fuel plant could create up to 20 times more employment than that created by a coal — or oil-based plant (Van Loo and Koppejan, 2008, p. 1). The biomass industry thus has a positive impact on the local economy.

Another very important aspect of biomass-based energy, fuel, or chemicals is that they reduce reliance on imported fossil fuels. The volatile global political landscape has shown that supply and price can change dramatically within a short time, with a sharp rise in the price of feedstock. Locally grown biomass is relatively free from such uncertainties.

Based on heating rate, pyrolysis may be broadly classified as slow and fast. It is considered slow if the time, theating, required to heat the fuel to the pyrolysis temperature is much longer than the characteristic pyrolysis reaction time, tr, and vice versa. That is: [1]

Biomass comes from a variety of sources as shown in Table 2.1. Loosely speak­ing, these sources include all plants and plant-derived materials, including

livestock manures. Primary or virgin biomass comes directly from plants or animals. Waste or derived biomass comes from different biomass-derived prod­ucts. Table 2.1 lists a range of biomass types, grouping them as virgin or waste. Biomass may also be divided into two broad groups:

• Virgin biomass includes wood, plants, and leaves (ligno-cellulose); and crops and vegetables (carbohydrates).

• Waste includes solid and liquid wastes (municipal solid waste (MSW)); sewage, animal, and human waste; gases derived from landfilling (mainly methane); and agricultural wastes.